One of the distinguishing characteristics of mammals is the ability of the mothers to produce milk in their mammary gland. The main function of mammary gland in animals is to provide milk for their offspring until their offspring develop their ability to eat other kinds of foods. The breast, where the mammary gland is located contains sacs in which the milk is being produced. The sac releases the milk into the ducts which flows until the end which is called the nipples. The locations of the mammary glands on animals vary depending mode of the nutrition and/or the position of the offspring. It can be related to evolutionary pattern.